In recent years, a secondary battery such as a lithium-ion secondary battery or a nonaqueous electrolyte secondary battery has been developed as a battery having a high energy density. The secondary battery is expected to be used as a power source for vehicles such as a hybrid automobile and an electric automobile, or as a large-sized power source for power storage. When the secondary battery is used as the power source for vehicles, the secondary battery is required to achieve rapid charge-and-discharge performance and long-term reliability or the like in addition to the high energy density.
Lithium ions and electrons rapidly move through an electrolyte and an external circuit respectively between a positive electrode and a negative electrode which can insert and extract the lithium ions and the electrons, to enable to perform rapid charge-and-discharge. The battery capable of performing rapid charge-and-discharge has the advantage that a charging time is considerably short. When the battery capable of performing rapid charge-and-discharge is used as the power source for vehicles, the motive performances of the automobile can be improved, and the regenerative energy of power can be efficiently recovered.
A carbon-based negative electrode using a carbonaceous material such as graphite as a negative electrode active material is used as a negative electrode capable of inserting and extracting lithium ions and electrons. However, when rapid charge-and-discharge is repeated in a battery including the carbon-based negative electrode, dendrites of metal lithium may precipitate on the negative electrode. The dendrites of metal lithium may cause an internal short circuit. Therefore, when the rapid charge-and-discharge is repeated in the battery including the carbon-based negative electrode, a concern is raised that heat generation and ignition may occur.
Therefore, a battery including a negative electrode using a metal composite oxide as the negative electrode active material in place of the carbonaceous material has been developed. In particular, in a battery using a titanium oxide of the metal composite oxide as the negative electrode active material, the dendrites of metal lithium are less likely to precipitate even when rapid charge-and-discharge is repeated as compared with those of the battery including the carbon-based negative electrode. The battery using the titanium oxide has more stable rapid charge-and-discharge and a longer life than those of the battery including the carbon-based negative electrode.
However, the titanium oxide has a higher (nobler) potential relative to lithium metal than that of the carbonaceous material. In addition, the titanium oxide has a lower theoretical capacity per unit mass than that of the carbonaceous material. For this, there is a problem that the battery including a negative electrode using the titanium oxide as the negative electrode active material has a lower energy density than that of the battery including the carbon-based negative electrode.
For example, the potential relative to lithium metal of a lithium-titanium composite oxide such as Li4Ti5O12 is about 1.5 V (vs. Li+/Li) or more. On the other hand, the potential relative to lithium metal of graphite is about 0.1 V (vs. Li+/Li) or more. Here, the potential relative to lithium metal of the titanium oxide is caused by an oxidation-reduction reaction occurring between trivalent titanium ions and tetravalent titanium ions when lithium ions are electrochemically inserted and extracted. That is, the potential relative to lithium metal of the titanium oxide is inherent to the titanium oxide, and the lowering of the potential is electrochemically limited. Therefore, it is substantially difficult to lower the potential relative to lithium metal of the titanium oxide to improve the energy density. Since the potential of the titanium oxide with respect to lithium metal is high, a battery including a negative electrode containing the titanium oxide secondarily allows stable rapid charge-and-discharge.
The theoretical capacity per unit mass of the lithium-titanium composite oxide such as Li4Ti5O12 is 175 mAh/g. On the other hand, the theoretical capacity per unit mass of graphite is 372 mAh/g. Furthermore, the titanium oxide has less sites capable of inserting lithium ions in its crystal structure than those of the carbonaceous material, and is likely to stabilize lithium ions in its crystal structure. Therefore, the ratio of the actual capacity of the titanium oxide to the theoretical capacity thereof is lower than the ratio of the actual capacity of the carbonaceous material to the theoretical capacity thereof.
From the above, the energy density of the battery including the negative electrode containing the titanium oxide is remarkably lower than that of the battery having the carbon-based negative electrode.
In view of the above, a new electrode material containing titanium and niobium has been studied. In particular, in a monoclinic niobium-titanium composite oxide represented by TiNb2O7, tetravalent titanium ions are reduced to trivalent titanium ions and pentavalent niobium ions are reduced to trivalent niobium ions when lithium ions are inserted. Therefore, this monoclinic niobium-titanium composite oxide can maintain the electric neutrality of a crystal structure even when many lithium ions are inserted, as compared with the titanium oxide. As a result, the monoclinic Nb—Ti composite oxide represented by TiNb2O7 has a high theoretical capacity of 387 mAh/g.